(1) Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell which is composed of a positive electrode containing a lithium-contained complex oxide, a negative electrode containing material that can absorb and desorb lithium or lithium ion, and a separator filled with an organic electrolyte.
(2) Description of the Related Art
In spite of the advantage of high-density energy, this type of cells have the following disadvantages: When the cells are externally short-circuited, large short-circuit current runs inside the cells. As a result, Joulean heat is caused, and the cell temperature rises to an extreme level. This might cause the organic electrolyte to react with the electrodes, especially the positive electrode, and to burn. In order to secure the safety of the cells against such an extreme temperature increase, the current supply is shut down by making the separators composed of polyolefin fine porous films melt down, thereby closing their fine pores. However, the films are as thin as 25 to 35 .mu.m, so that they tend to be damaged by the unevenness of the electrodes, thereby causing the cells to be slightly short-circuited. This leads to the self discharge.
In order to prevent the separator from being damaged by the unevenness of the electrodes, it has been suggested to use polypropylene (hereinafter PP) or polyethylene (hereinafter PE) that has high molecular weight for the separator, thereby enhancing the piercing strength of the separator. However, the separator containing PP or PE that has high molecular weight does not melt down until the cell temperature reaches a high level because the melting point of the separator is also high. This might cause the cell temperature to rise extraordinarily, or the shut-down characteristic of the separator to be deteriorated.
It has also been suggested to use a blend polymer fine porous film which contains both PP and PE for the separator, in order to enhance the piercing strength. However, the mere use of the blend polymer fine porous film containing PP and PE does not achieve an increase in the piercing strength and an increase in the shut-down characteristic at the same time. The reason for this is as follows: When the ratio of PE to PP is low, the fine pores of the film are not completely closed when PE reaches its melting point, whereas when the ratio of PE to PP is high, the piercing strength is low due to the influence of PE.